Research On Reliability Assessment Method For Structural Fatigue Of Semi-submersible Platform

Safety assessment technology of large floating structures is significant for successful designof platforms in the process of oil and gas exploitation towards deep-sea. Once structural damageaccident happened, it would lead to costly maintenance at least. And in some hazard conditions,it might cause serious economic loss, casualties and environmental pollution. Thus, it is veryimportant to keep structures of platform safety and reliability in the engineering design practices.Fatigue damage is one of the main failure modes of structural accidents for platforms. Forrationally assessing the safety and reliability of structural fatigue life for semi-submersibleplatforms, the paper carried out researches on strength models of structural damage, calculationmethods of fatigue loadings, assessment methods of fatigue life, uncertainty estimation of fatigueloadings and prediction of fatigue reliability.At the beginning, a new fatigue crack growth model called tangential model was proposed toinvestigate structural fatigue life reliability of semi-submersible, which is based on thecharacteristics of different phases in fatigue crack propagation and developing trends of fatiguelife assessment method during recent years. The new model has four parameters, takes stressintensity factor range as driving force and belongs to “single parameter model”. Then, this newmodel was employed to analyze test data of fatigue crack growth for four different materials. Thecorresponding parameters were obtained and it was proved that the model has well applicability.Cumulative fatigue damage (CFD) and fracture mechanics methods were taken to calculate thefatigue lives. The differences among S-N curve method, Paris method and tangential modelmethod are compared with each other, and the influence of initial crack length was investigated.Secondly, wave loading, hot-spot stress and fatigue life of typical structural joints of thesemi-submersible were calculated by spectrum analysis method (SAM) and design wave method.The calculation procedure of SAM was presented and the software interface between loadsprediction and structural analysis was coded. The principal wave loads and correspondingcharacteristic parameters of the design waves were recommended. After that, the paperinvestigated the effects of long-term exceeding possibility level of fatigue loading, shapeparameter of Weibull distribution, short-term sea state, wave spectrum, wave statistics andcorrosion mode on the fatigue lives of typical structures of the semi-submersible.Thirdly, for investigating the main uncertainties of wave loads during the assessment offatigue life, a set of measures for estimating wave load uncertainties were developed. Theuncertainties of wave spectrum, wave height, zero crossing period, transfer function of waveloads, occurrence probability of sea states and wave direction were obtained based on dataregression, experimental comparison and simulative calculation.At last, limit state functions of fatigue life by fatigue cumulative damage method andfracture mechanics method were built and fatigue life reliability index were calculated withconsidering various uncertainties.In the paper, some aspects such as crack propagation model, uncertainty analysis methods offatigue loading and associated results, some important parameters in fatigue loading and designwave parameters might have innovative meanings in the field of fatigue reliability analysis ofoffshore structures, and which might have certain instructive effects for structural design, safetyand reliability evaluation of semi-submersible platforms.